The present invention relates to a fluid flow control system which is especially useful in providing a rate of fluid flow in a slave path which is directly related to the rate of fluid flow in a master path, and which is particularly useful in the control of fuel gas in a flame atomic absorption spectrophotometer.
In atomic absorption spectroscopy, the measurement of the absorption of a radiation beam at a characteristic resonant spectral line for a particular element yields a measure of the concentration of that element in an original sample solution. Presently, one of the most common techniques for atomizing an element for purposes of the absorption measurement is by introducing a liquid sample solution of the element of interest into a gas burner wherein droplets of the solution are vaporized and the elements ultimately atomized, so as to form in the path of the apparatus radiation beam, a substantial quantity of the element of interest in its atomic state. A sample light beam, which originates from a line-emitting light source, such as a hollow-cathode lamp and which includes a resonance line of the element to be measured, is directed through the flame. The desired element in the sample absorbs the resonance lines characteristic of the element and the emerging light beam is directed to a monochromator and thence to a detector which measures the degree to which the desired element absorbs the resonance lines of the sample beam. This absorption degree represents the amount of desired element in the sample substance.
In such spectrophotometers, in order to produce a flame which has a high enough temperature for the best measurement results, for certain elements it is preferred to use acetylene gas as a fuel and to use nitrous oxide (N.sub.2 O) as the source of oxygen for the combustion of the acetylene gas. In order to initiate combustion in a safe manner, it is necessary to begin combustion of the acetylene gas using air as the oxygen source, and to then switch over to the nitrous oxide after the acetylene gas flame is ignited and stable.
However, upon switching over from air to nitrous oxide, it is necessary to essentially double the flow of acetylene gas to the flame in order to maintain the proper ratio of fuel to oxidant. It is also desirable to carefully regulate the ratio of fuel to oxidant by means of valves in the respective fuel and oxidant lines, and to avoid having that mixture adjustment altered detrimentally by the changeover from air to nitrous oxide and the doubling of the acetylene gas flow. Heretofore, this maintenance of fuel oxidant mixture during changeover from air to nitrous oxide has been a difficult problem.
Accordingly, it is one important object of the present invention to provide a fuel flow control system which is operable to provide controlled fuel flow rates at two different levels depending on whether the oxygen source is air or nitrous oxide.
Another object of the invention is to provide a fluid flow system in which the flow in a slave path is made to follow the rate of flow in a master path.
Other objects and advantages of the invention will be apparent from the following description and the accompanying drawings.